Most of the acoustical measurements in theatres, concert halls and musical spaces are performed by using a single omnidirectional pressure microphone.

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Spatial analysis of room impulse responses captured with a

32‐capsules microphone array

A.Capra, A.Farina, A.Amendola and C.Varani andrea.capra@mac.com

Industrial Engineering Department University of Parma

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Most of the acoustical measurements in theatres, concert halls and musical spaces are performed by using a single omnidirectional pressure microphone.

Introduction

Used for reverberation time and other monophonic parameters

NO information about the arrival direction of the sound

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Aim of the research: visual and dynamic representation of the acoustical behaviour of the room under test (theatres, concert halls, etc.).

The dynamic vision of this behaviour could be useful for :

• Finding unwanted reflections (echoes)

• Evaluating the spectral content of those reflections

• Checking if the sound reinforcement loudspeakers are correctly located and aimed

Having at disposal directive microphones and aiming

them in different directions it should be possible to obtain

a chart of the behaviour of the sound in time and in

frequency

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First attempt: a rotating shotgun microphone

2008 - two similar opera houses chosen for the measurements:

Teatro Sociale

di Como Teatro Comunale di

Modena

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Measurements equipment:

- Omnidirectional source

- Edirol FA-101 sound card and a laptop for the recording of the sine-sweep test signal

- Sennheiser ME66 directive microphone mounted on a

Outline rotating table

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- Azimuth: 18 steps (20°) - Elevation: 8 steps (22.5°)

- Impulse responses derived from sweeps

by using Adobe Audition 1.5 and Aurora

plug-ins

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Some results

Teatro Comunale di Modena Source in orchestral pit

Point A

125 Hz 1000 Hz 4000 Hz

24 ms (direct sound)

The direct sound is rich of low frequencies for the diffraction of the pit

40 ms (first reflection)

The high frequencies arrive to the performer 16 ms after the direct sound

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Teatro Sociale di Como

S1 R

Source S1: the first reflection arrives from the back wall of the theatre after 40 ms

S2

4000 Hz

Source S2: direct sound at high frequencies is weaker than the first reflection coming after 56 ms from the proscenium arch

4000 Hz

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Dynamic polar plot in vertical and horizontal plane.

Teatro Sociale di Como – 4000 Hz

Reflection from the proscenium arch

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The new approach

How can we obtain lots of directive microphones with only one probe? Using an array of capsules!

2009: first prototype of a spherical array (32 capsules)

Expanded polyurethane (too much delicate for

handling!)

32 capsules for earing-aid

(poor quality)

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Eigenmike

^®

by mhAcoustics

• 32 high quality capsules

• Pre-amplifiers and A/D converters packed inside the sphere

• All the signals are delivered to the audio interface through a digital CAT-6 cable

• The audio interface is an EMIB Firewire interface based on TCAT DICE chip:

- supported by any OS

- 8 digital output(ADAT) + 2 analogue output - Wordclock

• The pre-amplifier gain control is operated by MIDI control

…in the meantime …

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The signal processing

The idea Synthesis of 32 directive virtual microphones in the direction of the capsules employing a set of digital filters

M = 32 signals coming from the capsules

V = 32 signals yielding the desired virtual microphones

Bank of MxV FIR filters

y

_v

(t) = x

_m

(t) ∗ h

_m.v

(t)

m=1

∑

M

Output signal of V mic.

Input signal from the m-capsule

Matrix of filters

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Traditional design of the filters

The processing filters h_mv are usually computed following one of several, complex mathematical theories, based on the solution of the wave

equation (often under certain simplifications), and assuming that the microphones are ideal and identical

In some implementations, the signal of each microphone is processed through a digital filter for compensating its deviation, at the expense of heavier computational load

outputs of the microphone array are maximally close to the prescribed ideal

responses This method also inherently corrects

for transducer deviations and acoustical artefacts (shielding, diffractions, reflections, etc.)

No theory is assumed: the set of h_mv filters is derived directly from a set of impulse response measurements, designed according to a least- squares principle.

Novel approach

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Virtual microphones synthesized for this research:

4^th ORDER CARDIOIDS Matlab script

•Inputs:

9 2048 samples of each IR

9 The number of virtual microphones 9 Directivity of each virtual microphone 9 Azimuth and elevation of each virtual

microphone

IRs Matrix inversion

Output: FIR filters matrix

✕ =

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The new session of measurements Two different kind of musical space:

Sala dei Concerti

(Casa della Musica - Parma) Teatro “La Scala”

(Milano)

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Equipment:

•Eigenmike

^®

probe as receiver

•Laptop as recorder

•“Lookline” Dodecahedron as omni-direcitonal source

•Sine sweep as test signal

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The post processing software

MATLAB script

Polar plot of the sound levels in the horizontal plane

Mesh map of the levels on a 360°x180° picture of the room

Dynamic plots

1

^st

step

Import of WAV files containing the IRs obtained from the

32 virtual microphones

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Frequency bands of analysis Impulse Response length

Pressure Level (dB) Root 2

Root 3

2

^nd

step

FFT processing

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The results

The probe is not calibrated with an absolute level: every point of measure has its own level normalization and

colour scale.

NEVERLES

Growing the distance between source and receiver the S lobe of the reflections becomes more relevant in

comparison with the direct sound.

1 3 2

24

24 virtual microphones for horizontal polar

1

2 32 3

32 virtual microphones for 3D map

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Teatro “La Scala”: 4 kHz – root 2 – 8000 samples

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The direct sound

500 Hz

1000 Hz Radiation of the

wooden stage

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The source is close to the receiver

The direct sound masks on the map the first reflection on the floor

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The first visible reflection comes from the column behind the source, followed by a reflection on the floor and by a reflection coming from the opposite direction of the first one.

The reflections

4 ms

57 ms

The sound bounces

between the columns of the proscenium arch…

… and then arrives the ceiling reflection.

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Some notes

• The shape of the theatre refocuses the sound in the point of provenience

• Lobes in the 500 Hz band: they are effect of the diffraction

• The reflections coming from the walls are displayed

with cold colours: big amount of absorption

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“Sala dei Concerti”: 4 kHz – root2 – 8000 samples

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The direct sound

500 Hz - the sound in this freq. band anticipates the direct sound. Is the effect of the radiation of the wooden structure of the stage

500 Hz

4000 Hz

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The first reflection from the floor is clearly visible, followed by the diffuse sound of the wooden diffusors.

21 ms

Strong lateral reflection (90°)

coming from a plane even surface.

On the opposite side that reflection is not

present because of an

absorbent curtain.

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After this reflection the sound appears quite diffuse in the room apart from a little bounce between the lateral walls.

97 ms

Strong reflection from the back of the room.

The effect is audible

also from the stage and

causes problems with

high repeated notes to

the performers

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Conclusions

Today we presented a new approach to the impulse response measurements. It permits the dynamic view of the impulse responses plotted on a panoramic picture (360°x180°) by using 32 high directive virtual

microphones.

Positive aspects

Easy visualization of undesired reflections

Easy way for locating their arrival direction

Easier way for correcting them

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This is a first step in the research, a lot of work has to be done:

• Calibration of the probe

• Creation of a plug-in (VST or Audacity-based)

• IP-camera for taking the panoramic pictures

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Thank you for the attention

andrea.capra@mac.com University of Parma